1. Field of the Invention
The present invention is generally related to nuclear fuel assemblies and in particular to reload fuel assemblies that minimize the possibility of damage during installation of the reactor internal structure.
2. General Background
Most of the pressurized water reactors in the United States use tall slender fuel assemblies of square cross section. These fuel assemblies stand side by side in the reactor core. Approximately two hundred fuel assemblies are arranged in the core to approximate a circular cross section. A full core has the general shape of a cylinder. The tops and bottoms of the fuel assemblies are positioned next to each other using locating pins or blocks. There is a nominal gap of usually 0.040 to 0.050 inch between assemblies. During refueling, the fuel assemblies are carefully positioned at the lower end fitting and are free standing until the reactor internal structure for locating the top of the fuel assembly is installed. The fuel assemblies have a structural end fitting at each end and six or more spacers more or less evenly spaced along the height(length). The outside dimensions of these end fittings and spacers are about the same. Having uniform outside dimensions was necessary in early fuel assembly designs which enclosed the whole assembly in a porous shroud. Although the shroud has not been used in some time, the design tendency has been to have all the outside dimensions the same. In at least one pressurized reactor design, the fuel assembly upper end fitting is slightly smaller to provide greater clearance to help with the insertion of fuel into the core. Clearance between fuel assemblies is normally a help with handling and core insertion. The fuel assembly is unrestrained at the upper end as fuel is removed from and inserted into the core and, with adequate lead in capability at the top end fitting, will move laterally if necessary to allow unrestricted passage of an adjacent fuel assembly. It is during installation of the reactor internal structure that the top of the fuel assemblies are the most vulnerable to being crushed. The internal structure is massive and will easily damage a fuel assembly that is out of position. A fuel assembly can be out of position by leaning toward an adjacent assembly and closing the gap between fuel assemblies. Nuclear plant operators have, on several occasions, crushed fuel assemblies during the installation of the internals structure. Dimensional inspection methods developed to address the problem have been unpopular because of the time required and difficulty making accurate measurements under twenty to thirty feet of water. The potential for damage to a fuel assembly depends on the available lead in capability designed into the fuel assembly and the internals structure, the design gap between fuel assemblies, and the accuracy of the alignment of the internals structure to the core as it is installed. The internals structure is aligned by guide pins during installation and special rigging is used to insure that the structure is level and lowered slowly to prevent wave effects. The lead in capability is usually limited by the size of the alignment pins and blocks. The gaps between fuel assemblies will allow enough of the assemblies to lean in one direction until assemblies are touching each other. The movement of the upper end fittings across the gaps can result in a situation of insufficient lead in to allow installation of the internals structure without damage to fuel assemblies. However, a gap between fuel assemblies is needed for fuel handling and fuel insertion into the core.